Non-Patent Literature 1 discloses a photodiode in which, on an InP substrate as a group III-V compound semiconductor substrate, an InGaAs/GaAsSb type II MQW structure which lattice-matches with the InP substrate is formed as an absorption layer. This MQW structure comprises 150 repetitions (150 pairs) of 5 nm thick InGaAs and 5 nm thick GaAsSb. In the photodiode disclosed in Non-Patent Literature 1, a cut-off wavelength of about 2.39 μm is achieved.
The above photodiode has a structure in which a cap layer located on the side opposite to the InP substrate with respect to the absorption layer has p-type conductivity while the InP substrate has n-type conductivity, and light is incident on the rear surface of the InP substrate.
Meanwhile, Non-Patent Literature 2 discloses wavelength dependence of responsivity of a photodiode having the same structure as above. The responsivity is not flat with respect to the wavelength, but exhibits strong wavelength dependence.